1. Field of the Invention
This invention relates to a method for the production of maleimides. More particularly, this invention relates to a method for production of maleimides by ring-closure imidation of maleinamic acids.
2. Description of the Prior Art
Maleimides are compounds useful as raw materials for synthetic resins, medicines, and agricultural chemicals. Researches after methods for their production have long been under way. The most popular method of them all effects the production of maleimides by the dehydration cyclization of maleinamic acids with a dehydrating agent such as acetic anhydride. One version of this method is disclosed in U.S. Pat. No. 2,444,536. This method effects the production of maleimides by causing maleic anhydride to react upon amines thereby forming maleinamic acids and dehydration cyclizing and, at the same time, imidating the maleinamic acids in the presence of acetic anhydride and sodium acetate. This method, however, has the disadvantage that the imidation requires expensive acetic anhydride to be used in at least an equivalent relative to the maleinamic acid and the separation and recovery of the formed maleimide from the imidation reaction solution necessitates use of a large volume of water and, as the result, entails disposal of a large amount of an acetic acid-containing effluent at great expense. Thus, this method may well be called a too expensive method for commercial production of maleimides.
A method which has no use for such a chemical dehydration agent as acetic anhydride is disclosed in British Pat. No. 1,041,027. This method effects the production of maleimides by thermally dehydrating and cyclizing maleinamic acids in conjunction with a solvent such as, for example, toluene, xylene, or chlorobenzene having a boiling point exceeding 80.degree. C. and serving as a diluent and an acid catalyst such as sulfur trioxide, sulfuric acid, or ortho-phosphoric acid, and distilling the system thereby causing azeotropic expulsion of the consequently formed water in conjunction with the solvent. As compared with the method which uses acetic anhydride, this method proves advantageous in that it does not require use of a large amount of such an expensive dehydrating agent as acetic anhydride and further that the formed maleimides are separated and recovered with ease. This method nevertheless has the disadvantage that the yield of the imidation is low as compared with that obtainable by the method using acetic anhydride. This disadvantage is logically explained by a postulate that compared with the method which effects the imidation by the use of acetic anhydride, the method which effects the imidation by performing thermal dehydration in the specific solvent as described above involves a high reaction temperature and, therefore, tends to induce secondary reactions and inevitably manages to produce maleimides abounding with inpurities and further that since maleimides are thermally unstable, the maleimides produced at all are degenerated during the course of the reaction.
There is another method which, as disclosed in Japanese Patent Laid-open No. SHO 53(1978)-68,700 and Japanese Patent Publication No. SHO 57(1982)-42,043, comprises causing maleic anhydride to react on amines in the presence of an organic solvent thereby forming maleinamic acids and subjecting the maleinamic acids as held in a state not isolated from the reaction system to dehydration and cyclization in the presence of such an aprotic polar solvent as dimethyl formamide or dimethyl sulfoxide and an acid catalyst. By this method, there is offered recognizable improvement in yield as compared with the second method described above. This method, however, has these disadvantages, that the cost of production of maleimides is high because expensive and highly toxic aprotic polar solvent such as dimethyl formamide is used in a large amount, that the solvent such as dimethyl formamide is degenerated by the action of an acid catalyst used in the reaction and, therefore, the solvent is lost greatly, and that since the aprotic polar solvent used in the reaction has a high boiling point, the solvent is removed from the produced maleimides with great difficulty.
Yet another method has been proposed (as in Japanee Patent Laid-open No. SHO 47(1972)-27,974) which effects the imidation of maleinamic acids by directly heating the acids in the presence of an acid catalyst. Unfortunately, this method is not free from the following disadvantages that a large amount of resinous impurities is produced in the reaction vessel resulting to obtain maleimides in low yield, and that, moreover, the crude maleimides inevitably contain by-produced maleic acid in great deal.
In addition to the disadvantages described above, these methods for the production of maleimides are invariably attended by the essential requirement that since by-products occur in not negligible amounts during the course of imidation, the produced maleimides should be purified in order to acquire high purity at great expenses.
As means of purification such impure maleimides, there have been proposed a method which comprises pouring the reaction solution into a large amount of cold water thereby inducing precipitation of crystals, separating the crystals by filtration, washing the crystals with a large amount of water or washing them with a dilute aqueous solution of sodium carbonate or aqueous solution of sodium hydroxide, and thereafter drying the washed crystals (U.S. Pat. No. 2,444,536 and Japanese Patent Laid-open No. SHO 55(1980)-149,253) and a method which comprises neutralizing the reaction solution containing a maleimide in an organic solvent with a dilute aqueous solution of a weak alkali and washing the neutralized reaction solution with water thereby separating the organic solvent therefrom (Japanese Patent Laid-open No. SHO 53(1978)-68,770).
In accordance with these methods, however, it is difficult to obtain maleimides of high purity because the by-products which occur during the course of imidation are as insoluble in water as resinous substances and maleimides. Since these methods involve use of large amounts of water, they inevitably entail the problem of safe disposal of large amounts of washings. They are deficient in practical utility and prove disadvantageous from the economic point of view.
An object of this invention, therefore, is to provide a novel method for the production of maleimides.
Another object of this invention is to provide a method for inexpensively producing maleimides in high yields by the ring-closure imidation of corresponding maleinamic acids effected by a safe and simple procedure.
Still another object of this invention is to provide a method for the production of maleimides of high purity.